System for visually checking alignment of computer-tracking loop circuitry



P 0. 1956 D. v. FRANKE SYSTEM FOR VISUALLY CHECKING ALIGNMENT OF COMPUTER-TRACKING LOOP CIRCUITRY Filed June 2 19 12 sheets-Sheet l 4: seem bar/7 M04 T I 78297172 zm/eeree wvserse EH04 lean M a 4 w n 5 s E ME WW u mu Em L lW g Hw WM E .K n MN 5 a T0 Y r N? E 5 F N m W E V VA A I w W pm a w G 5 M 5 I H A #6 6 e k M MW W m m H e .P 9 s s M e 0 MW w a w 3 m a w w A ril 10, 1956 D. v. FRANKE SYSTEM FOR VISUALLY CHECKING ALCIGNMENT Filed June 2' 1952 OF COMPUTER-TRACKINu LOOP CIR UITRY 12 Sheets-She s: 2

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r I I l I lllullllllllllllll'll 7 mm/75 v. FfH/V/(E w 74% ATmRNEYS United States Patent SYSTEM FOR VISUALLY CHECKING ALIGNMENT OF COMPUTER-TRACKING LOOP CIRCUITRY Dallas Valo Franke, Redondo Beach, Calif., assignor to Gilfillan Bros. Inc., Los Angeles, Calif., a corporation of California Application June 2, 1952, Serial No. 291,139

11 Claims. (Cl. 343- The present invention relates to improved means and techniques for visually checking the alignment, or predetermined ideal course lines and glidepaths as computed electronically in an automatic ground control approach (AGCA) system.

More specifically, the present invention relates to means and techniques whereby the ideal glidepath and course line which are computed electronically, using the apparatus described and claimed in my copending application, Serial No. 266,001 filed January 11, 1952 for determining the deviation of aircraft from such course line and glidepath, may be observed visually on a cathode ray tube.

In my last mentioned copending application, means are shown for developing a voltage characteristic, the intensity of which is representative of the range of an aircraft being tracked, and also, means are disclosed for combining such electrical characteristic with voltages representing the angular position of an antenna beam scanning through the aircraft approach zone for purposes of developing an alternating voltage, the cross-over points of which represent the ideal glidepath or course line, as the case may be. The locus of these cross-over points constitutes the glidepatb and course line respectively.

In developing such alternating voltages, non-linear circuits, such as thyrite, are used to compensate for the fact that the situs of the radar antenna is adjacent to the aircraft landing field and is thus not coincident with the aircraft touchdown point.

The present arrangement contemplates a switching operation whereby the range tracking and computing system is essentially reversed, such that the computing system which is normally supplied with range information from the range tracking unit, is fed a voltage representing the angular position of the antenna beam. The computing system in such instance generates a "range voltage which is returned to the range tracking unit.

In such instance, when the computing system is thus reversed, the output from the range tracking unit is essentially a tracking gate or pip which is delayed in time, with respect to the system trigger, in an amount representative of the angular position of the antenna beam. This pip, in the form of video, is displayed on the cathode ray tube, and the resulting lines and dots, as the scan is continued, is compared to a fixed cursor or other visual reference lines, such cursor or visual refer ence lines being preferably produced using the. apparatus and teachings described in my copending application, Serial No. 266,002 filed January 11, 1952.

In an AGCA system, two antennas are used to scan the approach zone to an aircraft landing field, one of such antennas scans in the horizontal or azimuth plane, while the other antenna scans on a time sharing basis with the previously mentioned antenna, in the elevation plane. A voltage, termed the antenna beam angle voltage, is developed, the instantaneous magnitude of such voltage serving as a measure of the angular po- "ice sition of the azimuth antenna beam or the elevation antenna beam, as the case may be.

Also, the AGCA system incorporates means for automatically tracking the aircraft in its flight from a range ten miles remote from the aircraft touchdown point; and, during such tracking operation, a voltage, termed the range voltage, is developed, the instantaneoues magnitude of such range voltage serving as a measure of the distance of the tracked aircraft from the situs of the radar equipment. For obvious reasons, the situs of the radar equipment is adjacent the aircraft landing field and, of course, the situs does not correspond to the aircraft touchdown point.

The general purpose of the AGCA system is to develop, at the situs of the radar equipment, control signals for transmission to the autopilot approach coupler of the aircraft, so that such aircraft automatically flies along a predetermined glidepath (in the elevation plane) and along a predetermined course line (in the horizontal or azimuthal plane). These control signals cause the aircraft to fly up and down with respect to such glidepath, or to the right or left of the course line as required for flight along such glidepath and course line.

In developing such control voltages, an electronic computation is effectively made of the instantaneous location of the aircraft with respect to the predetermined glidepath and course line. In effecting such electronic computation, the predetermined ideal glidepath and course line are each represented by an electrical quantity, in this case, such quantity being an alternating voltage, the zero, or cross-over points of which, correspond to the ideal 'glidepath or course line, as the case may be.

In developing such alternating voltages, it is necessary to take into consideration the fact that the situs of the radar equipment is not coincident with the aircraft touchdown point.

The aforementioned electrical quantity, representing the glidepath and course line, is obtained, in general, by combining the aforementioned azimuth antenna or elevation antenna angle voltage as the case may be, with the aforementioned range voltage. By thus correlating the angular position of the antenna beam, with the 'range of an aircraft being tracked, the position of the aircraft is determined. However, certain compensation must be made for the aforementioned non-coincident relationship between the situs of the radar equipment and the aircraft touchdown point. This compensation is made using a non-linear circuit parameter such as thyrite for purposes of modifying the range voltage before the range voltage thus modified is used for comparison purposes with the azimuth or elevation antenna beam voltage, as the case may be.

In normal operation of the AGCA system, such ideal glidepath and course line as electronically computed and represented by the cross-over points of an alternating voltage, are not visible. The present invention is directed to means and techniques whereby the ideal glidepath and course line, as represented by cross-over points on an alternati'ng voltage are rendered visible on a cathode ray tube for purposes of checking the operation of the AGCA system for making adjustments, generally, and for alignment purposes.

It is therefore an important object of the present invention to provide improved means and techniques whereby the aforementioned results are accomplished.

A specific object of the present invention is to provide means and techniques, whereby a glidepath and course line, as electronically computed in the normal functions of the AGCA equipment, are rendered visible on a cathode ray tube by a simple switching operation requiring essentially no other circuit elements than those used in the normal function of the AGCA system.

Another specific object of' the present invention is to provide improved means and techniques useful in effecting a visible representation of a course line and glidepath which is comp ted electr ical y- Another specific object f the present invention is to pro ide a para fo visually present g an ideal glidepath and course line, using as a controlling pigment of circuitry, the non-linear thyrite, which is essential corn: putation of the ideal course line and glidepath the nor.- inal functioning of the AGCA equipment,

Another specific object of the present invention i to provide circuitry for visually presenting a and {:onrse line as computed .electronically,' eyen though the data for such purpose is obtained rada; egurgment .nh -sei ci en with th l shd n Point The features of the present inyention which are holieyod to be novel are set forth with partignlarity' appended la m Th 'n e en i s lf, oth as t ei hties: t oh a masses t h lh o ther thrthet 9i jeots and advantages thereof, may best on rstogd by tef r ne to the following d se ipt e taken ill sehhesliotl with t a e hehy ne wihss'in which:

Figure 1' shjws in schematie'form apparatus for spam h a p ach e to an air rstt lan in .l ld with related circuitry for producing a visual indication of the character illustrated in Figure ,o; also this appargtusserves to de e op i orm tion. such angle o tag le a ion hah se deo lshlsih volt ges and st e e y t s used n h. a ehtatie ar ed t lll rolle 'ari'preaeh (AGC s tem llustrat d t T F ure 2 .he zi eath sha -vo a e. ele a ion heat an le o t e a l a ih erte'd ele ation hear an: g1 voltage, lie slop d by e pa a sho in F ure l Figur 3 ho s a sle .5 o er n 0i r da s at nihg'ahd i i ting a an eme s n Fi ure l and er to i lus t e e o d i hi h th eas l relay volt.- age is available.

th i i t n wi h respe t to in s as A Figure 4 illustrates other voltages developodduring eye elie l opera on o h pparatu i lu trated in F gure l lgur 5 u rate more detail o the cathode l e terih me ns sho n in block t r Fi ure 1, and: cir u t y being e fe t to shi he d splays i F gure 6 sequ n ial y m one r n P n 0- o he other origin position 0-2 and from 0-2 to O1, etc.

Figure 6 illustrates the display obtained using the hp Ph htu ll t a d in gu 1, th e ation and azimuth ispl y being prod ced q a y on a time sharin basis. Figure 7 is a block diagram of an AGCA system entbodying features of the present invention which is supplied with certain information developed by the hp aratus ile lustr te n F u 1.

Figure 8 illustrates in schematic form circuitry of the video shaper which is indicated as such in Figure 7 and which is indicated in block diagram form in Figure 14.

Figure 9 illustrates the circuitry of the one-tenth cycle per seoond sawtooth generator which is also illustrated as such in block form in Figure 7, such sawtooth generator producing a sawtooth voltage wave of the character illnstrated in Figure 11, which is used during the smcalled -searoh funotion of the AGCA equipment, it being noted hat the ci cuit y o gu e 9 is lu t at n bl form Figur 10- Figure 10 illustrates in of llustra d in Fi ure 9- Fignrg 11 illustrates the sawtooth wave form developed by the apparatus illustrated in Figures 9 and 10.

Figure 12 illustrates in block diagram form the circuit- 11y of the AGCA tracking unit indicated as such in Figure 7, such circuitry being illustrated in detail in Figure 13.

Figure 13 represents in schematic form the circuitry of the AGCA tracking unit illustrated in Figures 7 and 12. Figure 14 illustrates in block diagram form the circuitry of the video shaper, the circuitry of which is illustrated block diagram form the circuitin Figure 8, the video shaper also being indicated as such in figure '7.

Figure 15 serves to illustrate the visual indication obtained of an aircraft being tracked with the bracketing index marks in one instance being limited by angle gating, while in the other instance being extended in the absence of angle gating.

Figures 16 and '17 illustrate in block diagram form different elements of the AGCA system and their func- Li nel inter-relatio sh p when he sy. rh is adju ted re: spec tively to use ground rate and air rate iniormation.

Figures 18A and 18B, interconnected as illustrated constitute Figure 18, which is a schematic representation of the apparatus in the angle tracking and computer unit illustrated as such in Figure 7, suoh circuitry of Figures 18A, 183 being illustrated also in block diagram form in Figure 19.

Fig re 0 il u t t s the ha aeter of he str ch. Yid'e 'hr deo s g al s c signal con ul. as in gene al a o g ted a e ha i g a ime duration equal iio the time during which radar 'ts" are being made' on an aircraft plus a fixed time interval in the order of 500 mierhse nds- Figure 21 illustrates the ng in the azimuth plane, with the radar equipment 10- elated a j c nt th r n ay center line nd in rel onshi to he d n. r t h figure h t i appreciating features of the computer illusll lmil 1. Figures-18A as a d 9- Figure 22 is useful in explaining the manner in which the azim th and ele a o he th v lt es a mania; house o compa ison w th a -.re erehee re as ti ae i h com r his Figure 2.13 llust a es the sl sh in hich he i cuit y the. treaties. u i and-semes e n t i m d fied'se 9 nrqrid a isual epred ti h en he ss ede ray b of hath the azimuth course l n and l t on g idepath. which a e emp ied by usin hy te e m nts in the nt rihal operation 0t thee mnute un 5 .liis re 2 4-illu t ates in schematic orm the circuitry of t'eutsersehera or use u i e pr duction or the alide'aath s me I runway cou s lin illus ra ed geometrical conditions exist? he and as ush in F gu e 6, suc .ireuitry bein i u t e in hla lt d a am in Figure 25,

Figure 25 represents in block diagram form circuitry of the cursor generator illustrated in schematic form in ighth 24 and incorporated in the unit designated *AGQA Qursor Generator and Artificial Aircraf unit n Figure 7.

Figure 26 illustrates a modified arrangement and is useful in illustrating certain concepts present in the AGCA system.

Figure 27 is a block diagram similar to Figure 26 and serves to illustrate the functional relationship of certain units of the AGCA equipment.

Figure 28 illustrates'the type of voltage variation produoed in the computing unit, the crossover points of which represent an'ideal glidepath and course line.

Figure 29 shows in block diagram form certain circuitry of the computer unit illustrated in Figure 1813 and is useful in illustrating the manner in which error traclting is accomplished, using a servo loop.

Figure 30 is useful in illustrating the time sequence of certain control signals and gates and echoes in relationship to the main bang or transmitted pulse in the range tracking unit.

I1 -l 9AGA system, the aforementioned antenna bean: angle voltage varies from 2 volts to 52 volts in accordance with the angular position of the antenna beam boingradiated, so that such voltage serves as a measure oi th position of the radia ed a enna beam.

omputing em in t ACi-CA rangement, ner ly temp a s he production hi all alternati h'velthse m t n nn b m hale volta setubined'with the aforementioned range voltage which is scanning beam may developed in the range tracking process, the'intensity of such range voltage serving as a measure of the range of the aircraft from the situs of the radar equipment. In checking such combination the antenna beam angle voltage is first level shifted and simultaneously the range voltage is modified by applying the same to a non-linear circuit element such as thyrite, and the range voltage thus modified is combined with the level shifted antenna beam angle voltage, either azimuth or elevation as the case may be at that particular time.

The computing system for producing such combination of level shifted angle voltage and modified range voltage is illustrated essentially with reference to Figures 18A, 18B, which together constitute Figure 18. As a result of such combination alternating voltages are produced, the zero or cross-over points of which represent the locus of an ideal glidepath or course line, as the case may be.

The present invention is directed specifically to producing visually on a cathode ray tube an ideal glidepath and course line as determined by such cross-over points. However, before describing this feature in detail, the AGCA system is first described generally, since the means and techniques presented herein, involve rearrangement of the elements of the range tracking unit and the computer unit from that which they assume with respect to one and other in the normal AGCA function.

Means shown in Figures 15 for producing information useful in producing visual indications and data for the range tracking and computing units in the AGCA function The apparatus shown in Figure 1 is'connected to the apparatus shown therein for producing visual indications' on the face of a cathode ray'tube 11 of the character shown in Figure 6.

In Figure l, the synchronizer '31 serves'to'generate timing pulses which are used to time the application of pulses to the transmitter'33 to initiate its operation. The transmitter stage 33, pulsed at a constant repetition rate of, for example, 2000 or 5500 pulses per second, consists of, for example, a magnetron. oscillator with a characteristic frequency of about 10,000 megacycles. The output of the transmitter stage 33 is transferred to either the elevation (el.) antenna 103 or the azimuth (az.) antenna 55, depending upon the position of the motor driven interrupter or radio frequency switch 101. The transmit-receive (TR) switch 97 prevents power from the transmitter 33 from being applied directly to the receiver 57. This transmit-receive switch 97, as is well known in the art, allows low intensity signals, such as a train of resulting echo signals received on the antennas 103, 55, to be transferred to the input terminals of the receiver 57. This deflection of energy from the transmitter 33 to the antennas 55, 103, accomplished by operation of switch 101, occurs at a rate of approximately two per second so that in effect the component antennas obtain four looks per second of the space scanned.

The resulting antenna beams are caused to move angularly i. e., to scan upon rotation of the shaft 93. The switch 101 is rotated twice per second, and while energy is being transmitted to one of the antennas 55, 103, the resulting electromagnetic beam projected into space is caused to scan such space. The means whereby such scanning movement of the projected electromagnetic beam is obtained may be of the type described in the copending application of Karl A. Allebach, Serial No. 49,910, filed September 18, 1948, now Patent No. 2,596,113, for bridge type precision antenna structure, which depends for its operation on the use of a variable wave guide type of antenna. This particular means, per se, forms no part of the present invention, and, so far as the aspects of the present invention are concerned, the antenna be produced by moving the entire antenna through a relatively small arc of a circle. Actu ally, in fact the azimuth antenna beam may scan first in one direction and then in the other, waiting after each scan while the elevation beam completes a scan in elevation. The azimuth antenna 55 scans a fixed hori zontal angle of 20, and is so placed as to includethe approach course to a given airfield runway. Vertical scan of the elevation antenna 103 is from minus One degree to plus 6 degrees.

While in any position during the part of the cycle in which the relay frequency switch 101 allows the flow of energy into the elevation antenna 103, the elevation antenna beam is electrically scanned in elevation. The position of the elevation antenna beam is measured by means of a variable capacitor 59, one plate of which is attached to the beam scanner of elevation antenna 103 and varied in acordance therewith, such capacitor 59 comprising one part of a capacitive potentimeter and contained in the angle coupling unit which may be of the type described and claimed in the copending patent application of Clarence V. Crane, Serial No. 212,114, filed February 21, 1951, now Patent No. 2,650,358. The angle coupling unit 85 thus used with capacitor 59 is useful in developing the elevation beam voltage represented as 61 in Figure 2.

Similarly, the angle in azimuth of the radiated azimuth antenna beam is measured by the angle capacitor 65 in the azimuth angle coupling unit 63A, operating synchronously with the scanner of the azimuth antenna 55. Such variation in azimuth angle voltage as a function of the particular angular position of the azimuth antenna beam is represented by cyclically varying voltage 63 shown in Figure 2. It is observed that these voltage variations 61 and 63 have portions thereof shown in heavy lines, and it is these portions which are used to effect control operations and which are selected by means mentioned later. Figure 2 also shows inverted azimuth elevation beam angle voltage as represented by the oblique lines 67.

Also coupled to the scanner of the elevation antenna 103 is the elevation unblanking'switch 67, which has one of its terminals connected to the continuous voltage source 91 for purposes of developing an elevation unblanking voltage gate, shown in Figure 4, so timed that its positive value corresponds to the time of effective scanning of the elevation antenna beam. The azimuth unblanking switch 65A is similarly coupled to the scanner of azimuth antenna 55 with one of its terminals connected to the continuous voltage source 658 for purposes of developing azimuth unblanking voltage (Fig. 4) so timed that the positive portion of such voltage corresponds to the time of effective scanning of the azimuth antenna beam. Relay switch 69 operates at substantially the same time as switch 65A, and synchronously therewith, and serves to generate the so-called az.-el. relay voltage or gate (Fig. 4), which is so timed that its positive portion begins at a time just prior to the beginning of the azimuth unblanking voltage and just after the end of elevation unblanking voltage, and which ends at a time just after the ending of the azimuth-unblanking voltage and just prior to the beginning of the elevation unblanking voltage, all as seen in Figure 4.-

Figure 3 shows a schematic diagramof the time re= lations involved in a scanning action which, typically, occupies a time in the order of one second. Forward progress of time is represented by clockwise motion about this diagram. The central circular region of Figure 3 marked N shows the time schedule of the scanning operations of the two systems, opposite quadratures being scanned by the same system but carried out in opposite directions. The shaded areas (each comprising approximately 10 degrees of the complete 360 degree cycle) represent the periods during which the transmitter 33 is switched by the switch 101 in Figure 1 from one antenna to the other antenna. Unshaded areas of region N represent the time periods during which one or the other at the anten as is n use. send n o t rad o frequency pulses and received reflected e ho signals from obi-sets within the field of coverage of the be m, Shaded areas indicate inactive periods during which switching take place, both antennas being momentarily isolated from the transmitter and receiver.

The inner annular region M of Figure 3 repr sents the time schedule of the related azimu h and elevation displays, subject however to pattern clipping described later, and corresponds to the cyclical variations of azimuth and elevation voltages re resented in Figure fhe outer annular region of Figure 3. marked L, shows the time schedule of currents through the various coils of a number of socalled az.-el, switching relays for effecting time sharing. The relay actuating urrent is obtained by the switch 69 (Fig. 2) operating in synchronism with the mechanism producing azimuth antenna beam scanning,

More specifically, in Figure l, the wave guide transmission line 79 leads from the transmitter 33 and receive ing system 97, 51. A Trjoint 7-1 divides this transmis-. sion line into two branches 73 and 95, leading through switch assembly 101 to the elevation and azimuth assen blies 103 and 55, respectively. These branches have suitably placed shutter slots which receive the rotating shutters 75 and 75A, respectively. 'lhese are mounted on the common drive shaft 93, driven by the motor 77, and have two blades each arranged in opposite fashion, so that when one antenna transmission branch is opened, the other will be blocked by its shutter. The shutter blades cover angles of approximately L, leaving open ings of 80 as required by re ion N of Figure it.

As mentioned previously, the same drive shaft 93 op, crates the two antenna beam seaming mechanisms repre: sented by the dotted lines 99, 79, and assumed'to be of the same construction as the above-mentioned Allehach application and built into the antenna assemblies. In the showing of Figure 1, the eccentric cams 83, 81 on shaft 93 operate the same scanning mechanism. Since each of the earns 83, 81 has one lobe, while. its associated shutter 75A or 75 has two lobes, one opening in the shutter will find the antenna scanning in one direction, the other in the opposite direction. -'Ihe azimuth and elevation unblanking switches 75A and 67 are shown schematically in Figure 1 as being cam actuated, being operated by the two-lobed cam 89, for purposes of establishing the unblanking or intensifying voltages represented in Figure 4.

The az.-el. relay switch 69 is operated by the cam 87 on shaft 93 to control current to the circuit switching relays, the junction of which is described hereinafter.

Radar echo signals, when received at the elevation an.- tenna 103 or the az muth antenna v55, as the case may be, are fed back into the, switch 101 and passed through the TR switch 97 into the receiver 57. Receiver 57 serves to detect the video-and after the video is amplified in the video amplifier stage 107, it is applied as SQ-fidlled normal video to the correspondingly designated leads 22 in both Figures 2 and 7. Such video, i. e,, radar video, derived from echo signals may be applied directly to the cathode of the cathode ray tube 11 shown in Figure 1 for purpo s of pr du ing vis al indications; or. u h normal video may first e sta dardized by applying the same to the video hatt r ndic t d a su h in the block di gram shown i Figure 7 and described in greater detail with re pec to Figure. 8- It underst od that ther m ans m y b u d for app ying he video to an intens y n r l elect de of thode ray t be a d y, for ex? mple, the. m ans and. he hotness-descri ed and claimed in th c ncedin applica i n of Landee et at, Serial N :2416 le ep mber 2.1, 19' L nd assigned to the same essisnee The a h de. ray tube 11 in Figure 1 h s a pairof magn ti deflec ion. .2215. 2%. so ar anged as to dehos he sso iate electr nic been: subst ntial y paral l to two mutu ly nem ndieular axes. o. th time base whi h is genera y, although not belly, horiz ntal as viewed y he ope a or, and he o al ed expansio a is h h is gene a ly ert al in genera each basic trigger pulse developed in SYl'lChIQ? nizer 31 (Fig. 2 is made to initiate a current wave of awtoo h f rm th ugh h t me base d flecti n oil 22B and a current wave of similar form through the associa ed xpan on d flec i n l 2A. th curr nt n each coil expanding approximately linearly with time and then returning rapidly to zero. Instead of a linear ariat his v riat n hey be loga th i n c ar c er as d sc bed i th co nding app i n of Homer 6. Tasker, Serial o- 1 .4168. filed Ju y .1 1. 5 an as: sign d to the sssign e as the pre ent appl a ion.

e r sult ng rate o s ch sawto thed ur ent i of rse the same s, o a fractio l mul pl o the pulse repetition rate of the transmitted radar pulses and occurs dur ng t e e pe tan pe iod r resulting e ho signals, I il be nder od th t. ect st de lec i n of a hod ray beam may e used ins ea o elec romagnetic efl ti n, appropria mo ficati n e ng mad in other parts of the equipment.

Su h sa l h cur ents app i d to the deflecti n coils 22B, 2%. howe er, ar m d at ch lower rate y curr nts f much lower pe dicity which are prod ced by th aforement o d be m angl oltages shown in Figure 2. Those portions of the voltage indicated in heavy lines in Figure 2 only are used to modulate the voltages on a. time sharing basis.

These voltages as represented by the curves 61, 63, may vary from plus 2 volts at one extreme of the scannine r nge to plu 52 olts a h other end hese p tl uler antenn beam an tag s s men on d pre lously a e us in dies; to modu te n amp tude of the sawtooth voltage waves developed at the sweep an plifier show in Fi u 2 and appli a a much h gh r epetition rate o t e expans on oil 2A, f r purposes of obtaining sci-ca led unld r. hora or uui mensioi rose nitu es in the a hode my di p ay, in ac ordance with th prin ples s for h n the c n lu ng pp a ion of omer G. 'lasker, Serial No. 680,604, filed Julyl, 1946, nd assign o the some ass e s the present anpl cation- On t e th an he amp i ud f the sa tooth voltage waves developed at the sweep amplifier nd applietl'to t e o her q adratur ly ct im g sed coil 22B is lilgewise modulated to a much smaller degree and in a d flerent manne for p p ses of orient io hus the amplitud o th c r ents applie to co 22A i au matically aried i a o d nce h nte n b am a l volteeeso tha th angl which any par i ular hode ray beam makes, o r sp nds, on a xpanded s le, to he ant nna b am lta e.-

The tu e ll i nde ed. l y op i .for p duc n ibl indica ns ly hen 's a in nsify ng volt e is app ed. o its rid 26, b ing ng ube ap o i a ely to cu f con i on, A relatively small additional video signal applied to the cathode 112C then strengthens the cathode beam, making it momentarily s l on t e s een as a h P s ion of wh c is e in d y h c r nts fl n at a p t c lar ome t n he e o deflec i n Qi s'ZZA 2 p p es o de op ng he aforemen ioned uita le d fl i ecu e ts n e cathode ay deflec ion cell's 2 A. 22. 3; the s eep g ng ci cuit ho in figure l i applied i h basi t iggers orig nating n the synchroeer 3. n appl ed to ead 10- 'Su h t gg r is appl ed to he lay mul i bra r a l k ng oscillator s age A. e ou pu f h c is f to. he weep generatim; multi ib etor s ge 1. 1A- n gative gati g o tage gener ted i he s a e lllA nd ed o he expan ion n time ase modula or stages 11%, 3A, respect ely, and tr ut th m is modulated form hrough. he aspension. an tim amp iers .1244 3 A, Ih ou put oi the amplifiers 124A, 125A in th term of ssentially trapezoidal waves of appropriate amplitude is applied to to the expansion deflection coil 22A and the time base deflection coil 228, respectively, causing current pulses of substantially linear saw tooth form in the coils. Expansion and time base centering circuits 126A, 127A, are also connected to the deflection coils. The modulator stages 112A, 123A, for purposes of modulation, receive az.-el. antenna beam angle voltages via switches in and n, respectively, of relay K1101.

With the relay unactuated (as shown) the elevation beam angle voltage appearing on the potentiometer resistance 134A is applied through switch m to the expansion modulator 122A, and through potentiometer resistance 135A and inverter 13513 and switch It to the time base modulator 123A. After completion of the elevation scan relay K1101 is energized through switch 69 breaking the elevation beam angle voltage connections just described, and connecting the azimuth beam angle voltage through potentiometer 136A and switch m to the expansion modulator 122A; and through potentiometer 137A, inverter 131A and switch n to the time base modulator 123A.

Thus the degree of modulation of sweep current, and hence the degree of angle expansion of the display shown in Figure 6 may be separately regulated for the azimuth display by adjustment of the potentiometer 134A, and for the elevation display by adjustment of the potenti ometer 134A; and the degree of modulation of the time base sweep current, and hence the apparent angle between the range marks and the time base may be sepa rately regulated for the azimuth display by adjustment of potentiometer 137A, and for the elevation display by adjustment of the potentiometer 135A.

The centering circuits 126A, 127A in Figure l are individually capable of two separate adjustments, one effective when relay K1102 is actuated (azimuth display) and one when the relay is unactuated (elevation display) to determine the position of the points 02, respectively, in Figure 6. Thus the origins of azimuth and elevation displays are separately adjustable, the centering circuits automatically responding to one or the other set of adjustments according to the energizing condition of relay K1102. A schematic diagram showing a centering circuit for this purpose is shown in Figure 5.

The deflection coil 22A in Figure is connected between a 700 volt positive supply and two parallel circuits, one leading to ground through tube V-1116, which is the final stage of expansion amplifier and .the other returning through choke coil 1.1101 and centering tube V-1117 to a 1,000 volt positive supply. The first of these two circuits feeds to deflection coil 22A, the periodically varying sweep producing component, while the second circuit provides a relatively constant but adjustable centering current component. The cathode resistor of centering tube V-1117 is made up of two parallel connected otentiometers 13 and 15, the movable contacts of which are connected respectively to the normally closed and normally open contacts of switch m of relay K1102. A switch arm is connected through grid resistor 17 to the tube grid. The grid bias, and hence the centering current through the tube and through the coil 22A thus depends upon the position of relay switch m and is determined by the setting of the potentiometer 15 when relay K1102 is actuated (azimuth display) and by the potentiometer 13 when the relay is not actuated (elevation display). The two displays are therefore separately adjustable on the indicator tube by means of the two potentiometers.

The time base deflection coil 22B is provided with centering circuit which is identical to that in Figure 5 and functions in a like manner, controlled by switch n of relay K1102. In fact, by appropriate changes of the numerals and lettering Figure 5 may be considered to illustrate the time base centering circuit. The potentiometer then provide separately adjusted ordinary eleva- 10 tion and azimuth displays with respect .to the horizontal positions.

It is noted that the preferred inter-relationship of the two displays in Figure 6 is such that the series of corresponding range marks of the .two patterns lie in a straight line so that the two aircraft images 38A, 39A always lie in a line just parallel to the range mark lines, one directly above the other.

The azimuth and elevation displays shown in Figure 6 are limited so that they appear as shown, such pattern clipping or limiting being produced by operation of the pattern clipper or limiter 40A shown in Figure 1. Such sweep limiter 40A forms, per se, no part of the present invention and may be the one described and claimed in the copending patent application of Raymond B. Tasker, Serial No. 212,163, filed February 21, 1951, and assigned to the same assignee, now Patent No. 2,663,868. in general, the output of sweep limiter stage 40A is a negative-going gating voltage 4013 applied to the first anode 19 of the cathode ray tube 11. Such negativegoing gating voltage 40B is used-for darkening, i. e., blanking out, the indications which may be otherwise visible. Such blanking occurs during undesired periods of sweep as now described specifically.

The azimuth display, which is preferably the lower one, is blanked or clipped or limited, above a horizontal line LM which extends parallel to the runway axis A and at a sufiicient distance above it to allow for expected errors in the azimuth angle of approaching aircraft. In the elevation (upper) display, a section is cut out or clipped, such section being below the horizontal runway axis 01G and to the right of a short generally vertical line KT. This line KJ is located just to the left of and parallel to the upper limiting sweep path 02L of the lower azimuth display. The region thus eliminated from the elevation display corresponds to space below the runway level.

Besides serving to produce this desired clipping in the visual display, the negative-going gating voltage 40B developed in the limiter stage 40A is useful in the automatic tracking system shown in block form in Figure 7 for limiting the time during which video is available in such automatic system. For that purpose gating voltage 403 is applied as shown therein to the Video Shaper" for purposes of limiting the time during which standardizfed video is produced in the manner described hereina ter.

As shown in Figures 1 and 7 the input to the sweep limiter 40A is: (1) a trigger derived from the basic trigger appearing on lead 10; (2) the azimuth and elevation angle coupling voltages on leads 18 and 20 respectively; and (3) the az.-el. relay voltage on lead 16. It is understood that this negative gating voltage 40B appears at variable times along the time axis depending upon the magnitude of either the azimuth or elevation beam angle voltage, whichever one at that particular time is effective.

The purposes of the switches 300A, 300B shown in Figure 1 are fully described in the above mentioned application of Homer G. Tasker and for the present instance may be considered to remain closed.

It is observed further in connection with Figure 1 that the sweep multivibrator 111A generates a positivegoing gating voltage 21 of a duration substantially equal to the time duration of the cathode beam sweep and such positive-going gating voltage is applied to the second mixer stage 23 to produce the wave form 25. This wave 25 comprises pulses of sweep frequency added to the longer 'azimuth and elevation gates which are developed in the first mixer stage 27 and shown also in Figure '4. This composite wave 25 is applied to the cathode ray grid 1126, bringing the tube up to the point of cutoff during each sweep. By this expedient the cathoderay tube is conditioned for producing visual indication only during those times when video signals are being expected.

The range marks 40, 41, 43, 45, 47, and 49, shown in Figure- 6, are developed by the range mark generator 41A (Fig. 1) in accordance with basic triggers applied to such stage from lead 10. The range marks developed in stage 41A are applied to the cathode 1120.

It; is. observed that the display shown in. Figure 6 ineludes. sectors defined by the so-called V-follower lines 50A, 51A and 52A, 53A,. which sectors are developed using the apparatus connected to the leads in Figure l marked Az. Servo Data No. 1,. A2. Servo Data No. 2, and El. Servo Data No. l and. EL Servo Data No. 2, allin accordance with the teachings in the above mentioned application of Landee et aL, Serial No, 247,616, filed September 21, 1951.

Also Figure 6% shows the glidepath course line 1491A.

and runway course line 150A. These two course lines may be developed electronically by apparatus described and'claimed. in. the copending application of Raymond B. Tasker and Burton Cutler, Serial No. 222,512, filed April 23', 195.15, and assigned to the same. assignec; or, preferable, these. lines are obtained using the cursor generator illustrated in Figures 24 and 25' herein.

Purpose and functions of apparatus The apparatus described hereincombiues the functionsof:

1. Aircraft acquisition 2. Automatic tracking 3'. Error computation and control signal" transmission.

The. controlled aircraft is' equipped with! suitable radio:

equipment: and an autopilot with: automatic approaclfc coupler.- This equipment; may be used as an: automatic: ground; controlled approach system (AGCA) for the! simultaneous guidance. of two: ormore' aircraft during; their approach; to; a given-.runway' adjacent to -wliicli.rad'ar-' 3 equipment: is located for scanning theapproach zone.

The radar system incorporates two antennas, one fori scanning: the approach: zone in. a. verticali plane,-. and the other antenna scans: the sameapproach. zone iniza hori- ZQnlifi plane. horizontal scan; is in the order of 20 lnaa-syst'ermof thiszcharacten. an: approaching; aircraft is first located by conventionahsearchradar; using-ion example -a' plan po'si' tion-i indicator (PPIK) and-isthen' directed by'ra'diocommunicationto-thecorrectposition-for entry into= a* pre determined: ideal glidepath (vertical plane) and course line (horizontal plane). The final approach along such ideal glidepathand course line is indicated upon-th'e face of; in cathodewray tube and= the; actual course of the air-- Vertical scan is from'zminus 191016 while 49 2 The range ofautom'atically controlled approach; is iron:v approximately eight milosfroni the given landingi field to a point of release fromthe system known touchdown.- This point of release, or touchdown,- is- 6 an altitude? of approximately fifty feet above the given:

landing strip; and at such a position? dfi altitude that the pilot may assume control for the actual landing operation during the last few seconds of the landing Prior to the establishment of flight control of an ap preaching aircraft, communication between the: instaliation and pilot of the incoming plane, niay be e'f footed via a conventional transmitter receiving system in the VHF band in the region of 240 megacycles'.-

Briefly, inoperation: ofthe AGGA system; the radar operator,- using the display or: the conventional search radar (PPI') tracks the aircraft to a proper p'o'si' tionoltitude of the AGCA- final approach. Theenti y into the AGCA-systern is alonganon-course approach line" at adistance ofapproximately ten miles: and at an have non of approximately 2800feet'ab'o've-the the" airem;

in. tho-meantime; the radarequipment; being ehe'fgiiedi' is in its Search function or conditionin Which-a slow searchswcep voltage is periodically developed for' searching aradarecho from the approaching aircraft. As" a matter of fact; coincidence of a radar echo frond s'u'cli aircraft with such: slowsearcl'u sw'ecpvoltage notifies thesystem of anapproachingaircraft; and thereupon the tracking unit; illustrated in Figure- 1-33- automatically switches fromsuch Search function or condit on to a "trac condition anddisplays the range and speed the incoming; aircraft. Simultaneously; upon-2 switching fromsuch -search to track function, thc AGGA"ttahs'ntit ter is turned on and a subcarrieron a transmitted waver containing a so called"channel select keyz isnransmitted 10' thei' approaching aircrafh- Ata given-range, oh upon directions from the ground via conventional radio tra n? mission,. the'- pilot of-= the approaching aircraft reirde'rs effective his airhorne decoder- (signal dnta convertefi by actuating switeht Actuation- -of such 1 switch starts='=the search. drive motor of=theairborne decoder, and the output of the=AGcA air hornet-receiver is -searchedfor an'AGGA SubcarrierE At intervztls ofzZSsseconds; the-AGflA -ground transmittertis" automatically interrupterl for a one-secondtpr'iod! This interruption: constitutes= interrogatidr'r,

If uponutheflinterrogation the airborne dcoderr-h'as located theitran'smitteci subcarrier; the A signal interruption causes the d'ctector 'to scnd-a-45O0 cyle pert-second Ca1+ finmation signal ito -thwgtound-via =thewairborne' r transmit" crafttisavisuallycomparedwith' that of an ideal approaclie. 505 '3 This' confirmation signal is recei v'e'tl by the AGCA" suchddeal approach; i; e., ideal glidepath and' ideal coursm line being developed electronically by a so' called cursor' generator.-

Imprior art-systems of this=cliaractcr3 radio cornmuni cation is' used to' direct glidepaths andcourselines; but in accordance-with tlie present-invention, means are-provided for developingand transmitthtgto the aircraft, control signals* which are representative" of the deviation-of the aircraft ftom-sucli glidepath and course line for purposes ofmaintainingy or tending to maintain,- the fiightof such -ahcraft along such: glidepath and course line.

For accomplishing: such automatic control ofaircraft'; the AGGA system described herein is such-as'to receiveinformation fr'omconventional GC'A radar equipment a relativeto the range azimuth and elevation positions-of thesapproaching aircraft and'to' compare/these positions withan ideal predetermined'glidepath; 'I'h'eresult-Lof this comparison-,- in-the form of error signals; iselcctroru icallyrcomputcdand automatically-sent to th'econtrolld' aircraft via: very-:high frieqnency radiocommunication; AGCA-airbornetequipment receives this information r.(cor-'-- rection: signals) andjnterpretsr-it: inzthecfomrcoticonn'ol 1 voltages-z which are applied .-:to.' the aircrafts autopilot". approach; coupler.

ir g al'ongfl e 55; maticallyrswitches from its receivecahd ser-ves to energize relayfwindings to =apply--'a?' +28 vol-tvso-called control signal '-to-.-a commoir bush? the =ground equipment.

At the time the range-trackingsunit in Fignre-B-ahto-g sear-ch" function'toits trac function-as described -above,-- a so-calledtracking on" signal developed-- in the range-trackingzunitis applied to" the com'pu'ter unit illustrated-in FiguresdSAVand -183150" that a computerunitis conditionedtocomputcqhe-error; t ii'iany; ofcthe-air craft from th'e-ideal-glide'path 'and'ideal coursc line.-

Upon development of the confirmatiom control. signal resultingvtfrom confirmation the- AGGA =tr-ansmitter -istur-ned' on-to-. transmit to the aircraft the error signals computed by. [the unit-shownin FigurcsflSAt land-18M as well-=as--certainz other informat-ion:'- Sucha=errorsignals; iu-fii, azimuth-and elevation control signalsyarr-Well asasignal representative of the instantaneous range-of theaircraft}, is us'ed to modulatethe--sub carrier-5transmitted to -the aircraft, to provide the --autopilotwith correction" signals ion-on? course-approachand providing thevpilot" with visual'displayiinstantaaeous range from "touchdown informatiom- Thadataiincluding- :control'1signals for efiectingnlight' of-=the aircraftas well asotlier controllsignalsrare trans mitted from the ground to the aircraft by the use of a subcarrier on the transmitted wave.

The AGCA system as developed includes a frequency spectrum which encompasses a carrier width sufiicient for the control of six aircraft simultaneously. For this purpose, the AGCA carrier wave, transmitted in the region of 109 megacycles, includes a 30 cycle reference tone, at 3800 cycle voice band, and six positions for subcarriers, equally spaced from to 15 kilocycles upon the basic carrier, there being one sub-carrier for each of the six aircraft. This condition is represented in Figure 39 which illustrates the side bands on one side of the carrier wave only.

The 30 cycle reference tone originates in the AGCA transmitter mixer, and is used as a reference signal by an airborne decoder entering AGCA control. Such reference tone serves as a comparison for a 30 cycle phase shifted tone, included in all of the sub-carriers.

The voice band from 300 cycles to approximately 3800 cycles is included upon the basic carrier. This band is used to pass frequencies over the AGCA semi-private voice line upon the establishment of control, i. e., confirmation to the ground by the aircraft. Through a holding relay in the AGCA coder described hereinafter, voice communication between the ground installation and the aircrafts pilot is automatically available for a period of one minute after a wave off signal (release of ground control) is transmitted from the ground; or, communication may be held for an indefinite period by the ground operator by the actuation of a control switch.

The six sub-carriers, equally spaced, may be included in. the modulation of the basic carrier at frequencies from 5 to 15 kilocycles above and below the basic carrier frequency. In general, the AGCA coder circuitry illustrated in Figure 41 serves to modulate a particular AGQA channel sub-carrier with control functions of azimuth error, elevation error and other functions enumerated below. This unit. includes a sub-carrier oscillator, which develops the sub-carrier used in the particular control channel. The output of the oscillator is modulated by a square wave, generated within the coder unit, and the shifting of the frequency, amplitude, type, and symmetry of this modulation is indicated in Figure 40.

The control functions modulating each sub-carrier and the methods of modulation are as follows:

1. Azimuth control of the aircraft is effected by frequency modulating the 30 cycle phase shifted signal included on the particular sub-carrier.

2. Elevation control is obtained by using amplitude modulation by variation in symmetry of a square wave as indicated in Figure 40.

3. The pilot of the aircraft is provided with information as to his range from touchdown, using amplitude modulation by varying the frequency of the square wave as indicated in Figure 40.

4. A relay in the aircraft may be controlled from the ground for purposes of effecting voice communication and for that purpose amplitude modulation using 30% modulation by the square wave is employed.

5. So-called channel selection is provided using 70% amplitude modulation by the square wave.

6 Warning signals are transmitted to an approaching aircraft when his spacing to a preceding plane is below a predetermined minimum spacing and for that purpose, frequency modulation using variable deviation of the 30 cycle signal on the sub-carrier is employed, in such case a deviation of 180 cycles constitutes a warning signal.

7. Also a wave off signal may be transmitted to the aircraft, the wave oif being effected upon absence of 30 cycle modulation of the sub-carrier.

The AGCA system uses standardized video, i. e., the radar echo signals are shaped by measuring incoming radar echoes and utilizing all signals above a predetermined level to produce standardized pulses, such stand- 14 ardized pulses being of equal width and amplitude to as sure consistent tracking performance.

Inasmuch as the antenna beams, i. e., the azimuth antenna and the elevation antenna beams scan through space on a time sharing basis, there are intervals during the scanning periods when thereis no video. Moreover during one antenna scanning cycle radar hits, i. e., echo signals may be derived from a plurality of aircraft in the approach zone, some of the aircraft being larger than others and of course at different ranges from touchdown.

In view of these considerations, the AGCA system is provided with so-called range gated automatic gain control in the radar receiver, so as to maintain the gain of the receiver at a substantially constant level during scanning cycles.

For this purpose, the circuitry of the range gated automatic gain control measures the amplitude of radar echoes reaching the ground based equipment and controls the gain of the intermediate frequency amplifier in the superheterodyne type radar receiver, in an inverse relationship. This circuitry includes adjustable memory" and learning" characteristics whereby the intermediate frequency gain control is partially dependent upon remembered input and whereby the rate of response to new input amplitudes may be varied.

The AGCA system incorporates certain safety features, one of such features being termed control with warning, and results when the separation between any two tracked aircraft falls belowa minimum preset value. This control with warning? signal may be observed visually by warning lights at the ground equipment, and is also transmitted to the pilot of the overtaking aircraft. This.control with warning signal is derived in the so-called overtake warning and wave ofimnit, illustrated in Figures 29 and 30, from data supplied thereto from two range to-respond to AGCA control signals and the circuitry for this is illustrated in connection with Figures 18A and 18B.

A third safety feature which is related to the first mentioned safety feature involves an excessive overtake condition. When an approaching aircraft overtakes a preceding aircraft by-more than a pre-established distance, an error wave off signal, derived by the circuitry illustrated in Figures 29 and 30 is transmitted to the aircraft to release the autopilot from AGCA control and effects the transmission of a maximum fly up signal to the aircraft.

The aforementioned wave olf signals should not be confused with the normal wave off signal transmitted to the aircraft. In a normal approach an automatic wave ofi signal is transmitted to the aircraft indicating the return of flight control to the pilot for the final landing operation. One minute after the transmission of such normal wave off signal the particular control channel which causes tracking of the aircraft automatically returns to a standby condition so that it may automatically re-enter AGCA channel sequence cyclically.

It should be noted that a complete AGCA system includes, for each aircraft to be tracked in the approach zone, the following components: a range tracking unit of the character illustrated in Figure 13, a computer unit of the character illustrated in Figures 18A, 18B, a coder and sequencing unit of the character illustrated in Figure 41 with auxiliary equipment. These enumerated elements constitute a so-callcd control channel and means are provided in the AGCA system for placing each channel in the following conditions to perform the designated functions. These conditions and functions are:

1. Standby 2. Search 3. Track answers Means are providethfo'n automatlcally 'setfnhhing the ingnnit': tos'er'ld: the confirmation tracking an s'i iriiit operation of a plurality oh control-Mel's; .thus; as to the AGCA coder and the AGCA computer, 4 Tra etr: srtmingtwo control channels #6:, Channel No; 1 and ing'ofi the incothir'ig -airc'raft in range commeiice's' at this Ghannel Nah 2, ,updn actuation the-AGCA equi'nrndm' point and consists'of constantly revising the range at the" inChanninNo; 1 automatically goes into al -search con-- tracking gate so that it continues to encor'npassthea ditiom: While the equipn'tentim lranirel No:', 2 remainsp aircraft} in a standby conditiohr=,-- (ih'annet No; :1 thus awaits'- Range voltage proportionate to the delay of the trash the incidence" of an aircraft radar ect'm' within: the glide ing gateinrelationship to the system trig'ge'r is displayed" path' approachi The incidence of su'h anaechd and applied to other portions of the circuitry. The difcanses the'equipment'ofi (ih'annel Nmt f td'automzitteally' 16 feren'tiatibn of range voltage with respect to tithe fat-d: change from the search condition to the track conditibfi diices'a' speed voltage hich' is also displayed. andalso ,instit'fites aftgruu'fidi- 5mm in equi'p- During-the search cdn'dition, the ran e gateofment in ('Eha'nnel'N'O. Z stiH remains in a stafnilby'chndh' tracking unit has a width of approximately 2.2 r'ni otion;v h seconds. Confirmation of ground control by irico'inmg Means'tar provided foifipmvflflng; 15 aircraft-appliesa eontrol on signal to the tiakiiifi finitment in both Channels No. 1 and No. Z-from tracking to cau se'sixch unit" toatitornatically switch from its trick the same aircraft; al'ihtsugtn the eqtfipnrentzirr Channels function-- to its'cbfitfol function and to simui'tn 'ddgtf No: l and No. 2';maybothr simultaneouslys-controlf dif cause the tracking gate to be narrowedt'o approiriniat'ly' ferent While tiapkifigeaii-cl aft in the tracked 2i2' naicfoseconds; cgn-ditign; th pafficuim-channgbhgzflimgffdf dfiflfiyiji'gi 2i) Du'ring' P'GYiOd S CO'nt'lOl the range tracking the" instantaneous range: amtwpeedmtthe alrcraft being passes ra'n'ge information to the AGCA computer (Fi'g tracked' by that channel and= transinits sigtials of'intei urifSlSA, 18B) and,' iii d fl; a Via-8'0 $2 rogation' to'th'e aircraft. r wh'icli'= hotifies' the computer of the tir'rl'e at which video'i's" by merinmmihg irc aft; m h zgq ipmnfinzchanz. t e-making un'ifntaytrack video is limited by ah angie' the' approaching-zaircraft frol'm'falling withiir a pretletraforeinentiondchannels No'. 1' and No. 2, generates'a' mined'n'rinintnm*spacing gztlienrthezetlniinherit in fiihailnel range gate the'tir'h'e delay' of which" is directly prdflqi No." 2-'antornaticailyigoesiinto-a3wave elf -'c'onditibificit" ti'o'rlat t6"th'e iristiir'it'anedus range" of a tracked aircraft. functiomzahch transmits up signal tb-the This range gate is a 'ziplied'to the proper individtial'cliaji approachingfaircraftfl ..Otlier ;wae offsignals Maw-he nel'in""the"-overtalc'e warning andwav'e ofi'unitj'which transmitted by eithertChanfils Nofl or' Nof zg 'dpendingi creates a so eall'd safety gate of rnannall'y variable eitherontwheth'ei or'n'ot'f the particular aircraft-being: width, immediately following'the"trackedaircraft; tracked resporr'd's toFconu-ol sighals; and; a 'normal wave: The safety ages following each tracked aircraft are off signal is transmitted by Channels No. 1 add'No'lifsiic 5t) afiblid to'a conififonb'uiwhich is const'alntly nior'ii red ces'sive-ly as" the aircraft which they coxtesphndifi'gly by a coincidencedetector. coincidehce of thirla'n'g'e'j tracked reaches .tlrew tofichdown point: t h. ra er-eng neeredaircraft with 'tha'tof anothertriggrs In gcnerah'nthe rangetrackingmnit illusiratehfin Fig a saivtooth generator circuitry whose'outfiut is m as rea ure l3'performs 'the'rfunctions of aii'craftz acquisiiion ariil? for cor'it r'l' urposes. The degree of overtal't'e; ei'r' aircraft range tracking and displayst the ihs'fantaneotis pressed as an output voltage for eachchannehfis speed and rangf'of the trackedairctaffi: Iniahdit'roni thisf: urea ytwarelay confrolcircuits. The existence of an unit'prbdu'ces'aftrackingxon signal-:'af'tlie?time*-th -unitovertakchndihbriresults iri closing of a'so-calledover is swithedfroni asearchkonditibn toPit's track'condition'f take relay;" which provides suitable warnings" from a" Also"developed'sirrfthb traclringcunitris aisoicalledwidew common bus," Qrater degrees of overtake actuate the" on signal of the chhractercillustratedrihv Fi'gfiiaZOf fOrY so-calld' w'a've' olf'relay to cause the-transmission of notifying? el'nients in zrthe com'putet-tunif illustrated wave, off signals t6 e'aircraft; irr'Figtn-es 18A,:zl8Brofi1heqimefat'wliichlvideo -ispfesl tmae'fardaltee w'i 't'ur'e s" of tlieifis'fant invention, ent: Further; theztrackingaunitqdevclqp'm? Zmil'e pick: thWAGGN'sfit'eri?ifieofibbrates rnati's"shovvr'1 'iri"Fig'iifes 0E1 signal- -.when' thafl aclged aircraft miles-z; 181A?" 183 f6i' generating an alternating 'volt'a'ge repreof --tou chd0 wn so as-to r t=nfl cfigotiygfithconerationrpia 5 sfiting? afithe "'ro vbl't'age crossover points an idealf err-9r e: circuitry, in thecomputen 'glitlepath and=.idea1-course:line,- which :are so adjusted A; 3 HM w W, a toqcoincide twithwn actual physicalidal approachrtma" The operat ion. f tlielra pgestgrtigigi tjgk. chaggelis such given airfield. This glidepath and course line thus geh i Wh ni h, f' l h;9 r i i tie, email d-t f r-i hemrima "pu posev f; e e p n @ut i t k fie u' i ffi idw ea b'm li'ro g ahio tr4 t -.th i t m ai s afl- Whi a v'afiahl"dlyigiQkbiiirB $1 79 i fl lcii jc elevation and in mut rmai'l zeis e kcvisyallyx a s per second sawif'oo'th'vv av' illii'sfriif dqn' igfire'di J face gt a cathoderay.jube ypcn rearrangementog the developed by the 'circuimshpwn in Figure 10. Mg circuitry usoq'illlaflwgmaplishing the primary function of c d n egai a j the range s w th-ti h eloni s E lQ 8RQ S is l y' f vi o r pre nting an ir r f causes th track- Also the AGCA system includes a so-called artificial 

